A brake system of this type is known for instance from German published patent application 30 21 116.
The problems with brake systems of this type can be described as follows:
The basis for the following are anti-lock brake systems operating according to the return delivery principle. Conventionally, brake systems of this type are equipped with a non-self-priming pump, since vacuum must not be formed in the pump as soon as the low-pressure accumulator out of which the pump delivers fluid is exhausted. Also, the wheel brake should not be released excessively, which would result in the brake lining separating from the brake drum or brake disc, respectively. To operate a non-self-priming pump, an initial pressure is required which is generated by the low-pressure accumulator.
When it is desired to use an anti-lock brake system for traction slip control, this system must be capable of filling the wheel brakes to be able to build up a braking pressure which counteracts the drive torque. Therefore, the pump must suck in fluid out of a pressure-fluid supply reservoir. This usually requires a self-priming pump as the pressure-fluid supply reservoir does not provide any initial pressure.
Therefore, two pumps are provided in the brake system according to the above-mentioned publication which perform the various operations. It is clear that the result is a correspondingly complicated auxiliary-pressure supply system.
Admittedly, there are brake systems managing with one pump for both types of control. These brake systems are known as "open-circuit" systems. In this type of system, the pressure fluid which is discharged from the wheel brakes for reducing the wheel braking pressure is led to a supply reservoir, while the pump is constantly taking fluid out of the supply reservoir. The supply reservoir is filled with a sufficient quantity of pressure fluid which is delivered to a controlled circuit, from where a specific quantity of pressure fluid is put aside for the wheel brake as soon as pressure increase becomes necessary. In these systems, there is the risk, however, that braking pressure can no longer be developed in the event of failure of the outlet valve.
Therefore, the present invention is based on the object to provide a brake system which comprises a simple auxiliary-pressure supply system, which conforms to all safety aspects and wherein each brake circuit has only one pump.
In achieving this object, it is of importance that the opening pressure of the first valve is in excess of the atmospheric pressure. This is because once the brake force is discharged completely, the atmospheric pressure of roughly 1 bar prevails in the wheel brake and in the return lline. When the opening pressure of the first valve is in excess of approximately 1 bar, the pump is unable to prime, even if a vacuum is formed in the pump. Thus, the brake linings will not come off the brake drum or the brake disc, respectively.
It is important that the opening pressure of the first non-return valve is less than the initial pressure of the low-pressure accumulator.
When the brake system performs a brake slip control operation, the connecting line leading to the pressure fluid supply reservoir will be closed, and the pump takes in fluid out of the low-pressure accumulator via the first non-return valve.
The closing can be achieved in various manners. One possibility is to arrange for a non-return valve to shut itself. Other possibilities employ a change-over valve which is operated either by the pump pressure or by the master cylinder pressure. An actuation by the pressure in the low-pressure chamber is also possible, as well as by a mechanical actuation by the piston of the low-pressure accumulator.
The construction becomes particularly simple when both non-return valves are connected in series, with a closable connecting line to the pressure fluid supply reservoir branching off between the valves. The closing valve can be combined with the separating valve in a favorable manner.
According to a particular embodiment of the present invention, the brake system requires little space, is easy to assemble and affords low-cost manufacture.
As a result, the housing unit, termed as TSC-adapter hereinbelow, in which non-return valves and the two-way/two-position directional control valve are accommodated can be manufactured to be very compact and can be mounted into a pump unit of an anti-lock brake system. The pump unit operates according to the return delivery principle yet with a self-priming pump, at that location where one single suction valve is arranged if the system performs only an anti-lock function. Preferably, the TSC-adapter is screwed in exactly like the suction valve. An originally existing connection of the pressure line of the pump to the master brake cylinder will be closed or omitted during manufacture.
When a valve block comprising essentially only the inlet and outlet valves is replaced by a valve block which, additionally, comprises the separating valves and which is being built for other brake systems with traction slip control, a low-cost compact brake system with brake slip control and traction slip control is obtained by virtue of only a few innovations.
The present invention shall be explained in more detail in the following by way of nine Figures.